Low carbon formable and ageable alloy steels



United States Patent Ofiice 3,365,343 LOW CARBON FORMABLE AND AGEABLE ALLOY STEELS Milton B. Vordahl, Henderson, Nev., assignor to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey No Drawing. Continuation-impart of application Ser. No. 348,864, Mar. 2, 1964. This application Apr. 4, 1967, Ser. No. 628,292

3 Claims. (Cl. 75-125) ABSTRACT OF THE DISCLOSURE This invention relates to alloy steels, and in particular, to alloy steels which exhibit the desirable property of being quenchable to a relatively soft condition, wherein they can be readily worked and formed, and are capable of then being heat treated by aging at a moderate temperature to yield a material of substantially greater strength and hardness. In particular, the present invention relates to steels of the above-indicated type which are low in nickel content (less than about 3.5%) and are, consequently, considerably less expensive than other steels and alloys hitherto known which exhibit the property of being quenchable to a soft condition and then hardenable to an increased strength by aging at a moderately elevated temperature.

This is a continuation-in-part of my copending application, Ser. No, 348,864, filed on Mar. 2, 1964, now abandoned.

Other age-hardenable materials are known, but none are as inexpensive as the steels of this invention. For example, the alpha-beta type titanium alloys exhibit such properties, but these are expensive materials, costing three or four dollars per pound.

Age-hardenable, fully austenitic stainless steels are known, but in these, the response to age-hardening is not ordinarily of such great magnitude as to make the steel greatly more formable in the quenched condition than it is in its age-hardened condition, and these too are relatively expensive materials, costing about sixty cents per pound or more. Age-hardenable, fully austenitic stainless steels invariably contain substantial quantities of nickel and /or manganese, for example, at least about 7 /2 percent of nickel or about 12 percent of manganese. They require, moreover, either careful balancing of austenitepromoting elements and ferrite-promoting elements, or small additions of an element such as phosphorus, to develop age-hardening characteristics.

There are also martensitic (such as Stainless W and Type 17-4PH) and semi-austenitic (such as Type l7-7 PH and AM-350) stainless steels, which exhibit pronounced strengthening upon aging. However, all these require very delicately balanced compositions containing at least 4 percent of nickel or an equivalent amount of another austenite stabilizer, and all require complex cycles of heat treatment, involving such steps as double aging, sub-zero chilling, and cold working, all of which is quite incompatible with the main object of my invention, which is to afford a low-cost steel exhibiting marked response to an aging heat treatment.

Recently, there has been interest in the metallurgical arts in maraging steels that quench soft and are agehardenable, but these too are relatively expensive mate- 3,365,343 Patented Jan. 23, 1968 rials, containing at least about 9 percent, and usually substantially more, of nickel.

I have discovered that by making steels relatively low in carbon, with a low nickel content not exceeding about 3.5 percent, together with a small amount of titanium and a small amount of aluminum and/or silicon, the balance being essentially iron, I can produce a steel that is quite inexpensive and yet exhibits the desired property of quenching to a soft condition and being age-hardenable to a condition of substantially greater strength and hardness. Such a steel is exceptionally suitable for applications requiring a low-cost alloy steel, formable, and with a reasonably good strength-to-weight ratio. The properties of my steel are suited especially for such applications as shipping containers for use in railway, motor, or airfreight transport, for structural parts of railway cars, trucks and buses, and the like.

Reference is directed to Table I, below, which illustrates the chemical composition of the steels of this invention, as exemplified in a general range and preferred ranges of the chemical composition of these steels.

TABLE I General General Narrow Element Range Preferred Preferred Range Range (Cjarbon. 0.15% max. 8.088% max. g. %8% max.

opper. r I\lgickelnn anganese Titanium Aluminum. 'lieon... Chromium Cobalt Molybdenum 3 Ni-Mn-Cu. Al-Si 1 5% 1-3%.. 1-3

Iron In each case, balance, except for impurities in small amounts not detrimentally affecting the properties.

40 The primary object of the invention is to obtain heattreatment response in a steel as inexpensive as possible, and hence the emphasis herein is on the low-nickel steels. That nickel and aluminum will act together to provide strengthening on aging is known, but such a concept has never heretofore been applied to the production of lowcost, low-nickel steels such as those of this invention. Moreover, I find it advantageous, with lower nickel contents, to use added copper together with titanium and silicon toward the upper ends of the ranges stated above. The copper substitutes, to some extent, for the nickel, providing an austenitic matrix that can be transformed martensitically, while the controlled low manganese content is required to avoid brittleness and unworkability. The titanium and silicon form a compound or complex that contributes strength. Low contents of carbon are preferred, to preserve workability, At least some nickel is essential, to obtain heat-treatment response, but the use of more than about 3.5 percent is generally undesirable because of greater cost without corresponding benefits. Although such elements as chromium, cobalt and molyb denum can be tolerated in the rather great amounts indicated above, any advantage obtained from adding them is generally more than offset by the increased cost.

The composition and properties of certain alloy steels constituting individual embodiments of my invention are set forth in the following Table II.

TABLE II Alloy Cu Ni Ti Al Tensile Bend Ductility, (Progressive Bend Limit) Hardness as Queuched from 1,800" F.

Rolling Characteristics, Edge Rolling Characteristics, Surface Hardness as Aged, VHN 1 Hommmmmacnmwphwa w-mw a km @M00000000NON0 8-00 HDJMHHWCHMHWNNMNWHM 000Hl-NONH0000HOH0 HMNHQQMOOMHwHONv- M 1 After being Water-quenched from 1,800 F. and then aged for 1 hour at 1,000 F. or 3 hours at 900 F.

2 Also contained 8% Co, 4% M0.

The above data were obtained by using 30-gram, triplevacuum-melted buttons of the indicated compositions.

similar results with other alloys not within the scope of my invention, are presented in the following Table III.

TAB LE III Alloy Ni Al Hardness as Aged, VHN 1 Hardness as Quenehed from 1,800" F VHN Rolling Characteristics, Surface oocoooooooo crowoocooootcoo QZUDCDCIWWOOOJNCOQO k 000000M050CA70000 ew ssaeasr e 1 After bei rig water-quenched f1 om 1,800 F. and then aged for 1 hour at 1,000 F. or 3 hours at 000 F.

The balance in each case was iron, and all components used in melting the buttons were of the highest purity. The buttons were rolled at about 1800 F., to a thickness of about 0.05 inch, and then cleaned by pickling in dilute nitric acid solution. In the above table, the Vickers hardnesses as quenched from 1800 F. were determined upon specimens that had been heated to the indicated temperature for one minute, and then quenched in water. The indicated rolling characteristics, edge and surface, were determined by qualitative observation; in the table, the letter B indicates excellent and the letter G indicates good.

For purposes of comparison with the above alloys, 75

From the foregoing Table III, it will be apparent that certain alloys somewhat similar in composition to those of Table II, are not within my invention as defined above in Table I, and exhibit properties considerably inferior to those steels of my invention, either as respects rolling characteristics or as respects their response to heat treatment. In Table III, P and VP indicate poor" and very poor, respectively, and G and E have the meanings above-indicated.

Additional data concerning the response of the steels of this invention to heat treatment are presented in the following Tables 1V, V, and VI.

5 6 TABLE IV to 1800 F. and then aging for about 1 to 3 hours or more at a temperature of about 1000 to 900 F. Vickers Hardness After Heat Treatment While I have shown and described herein certain em- Alloy bodiments of my invention, I intend to cover as well any modification or change therein that may be made with- 1 2 3 4 5 6 5 out departmg from the sp1r1t and scope of the rnventlon. 317 575 250 548 249 545 I clalmi 361 670 363 36 36% 293 1. A steel conslsting essent1ally of about: up to 0.15 3% 28% 22% 2 83 percent carbon, 0.5 to 7.5 percent copper, 1 to 3.5 percent nickel, up to 0.75 percent manganese, 0.75 to 4 percent Heat Treatment, 10 titanium, up to 3 percent aluminum, up: to 3 percent siliigggj g? gf H t 1 000 F con, up to 18 percent chromlum, up to 20 percent cobalt, 3 hours 90 5 W 91 quenc or our a up to 8 percent molybdenum, the sum of triple the nickel 1min11teWaw1quench- ,0 content plus the manganese content plus the copper cong%% b o f quench ageforlhour at L000 or 1r tent being in the range 10 to 16 percent, the sum of the 23,2882? g igi fie gfi g ggem lhour (it 1 000 F or 0 content of aluminum and silicon being in the range 1 to 3 w q en percent, balance essentially iron.

TABLE v Heat Treatment Yield Strength Ultimate Elongation Reduction Alloy Hardness, at 0.2% Tensile in 0.5 inch, 01' Area, Solution Aging Aging VHN Offset, Strength, percent percent Temp., Time, Temp, ps.i. p.s.i. 1O-

F. Hours F.

1, 800 1 1, 100 132 157 7 17 1, 800 1 1, 100 133 161 s 4 1, 700 3 900 168 103 3 12 1, 500 1 1, 000 240 253 2 10 1, 500 1 1, 000 212 226 a l, 600 1 1,000 216 Broke l n Grips 1, 800 1 1, 000 197 21s 0 0 1, 600 1 1, 000 188 203 12 22 TABLE VI 2. A steel consisting essentially of about: up to 0.08 percent carbon, 0.5 to 6 percent copper, 1.5 to 3.5 persolution Aging Aging Hardness, cent nickel, up to 0.75 percent manganese, 0.75 to 4 per- Alloy Tgm 'l imc, Tgrp pq H cent titanium, up to 3 percent aluminum, up to 2 percent silicon, up to 2 percent chromium, up to 2 percent cobalt, 1 700 0 W 40 up to 2 percent molybdenum, the sum of triple the nickel 1:700 1 "5, 5" 5,55 content plus the manganese content plus the copper con- 58g 2 288 2g: tent being in the range 10 to 16 percent, the sum of the 1 1,500 653 aluminum content plus the silicon content being in the %,ggg g range 1 to 3 percent, balance essentially iron. 1:600 3 900 fig 3. An age-hardenable, low-cost steel consisting essenl tially of about: up to 0.08 percent carbon, 4 to 6 percent copper, 1.5 to 3.5 percent nickel, up to 0.50 percent manganese, 1.5 to 3 percent titanium, l to 3 percent aluminum, AS Table IV P the use of a lower sohmon 1 to 2 percent silicon, up to 0.5 percent chromium, up perature causes, 1n general, somewhat lower hardnessnn to 05 percent cobalt, up to 05 Percent molybdenum, the h solutlon'treated'andaged con'dltlon' AS Table V sum of triple the nickel content plus the manganese condicates, such lower hardness values are assoclatefit w1t tent plus the coppgr Content being in the range 10 to 16 lower strength and greater ductility. Table V a11 111 wages percent, the sum of the content of aluminum and silicon that hlgh strengths (over 200,000 p.s.1. y1e d strengt being in the range 1 to 3 percent, balance essentially iron. can be obtained, even with solution temperatures as low 5 as 1500 F. Table V also shows that with certain alloys, D References Cited e.g., Alloy 178-7, a considerable improvement in ductility properties can be obtained, at the expense of some UNITbD STATES PATENTS diminution in hardness and strength, by using a slightly 2,482,096 9 94 C arke 75--125 lower solution temperature. 2,694,626 11/1954 TaIlCZYH 7 -124 Table VI constitutes a study of the use of different w 2,715,576 8/ 1955 PaYSOH 75124 aging times and temperatures, for two different solution 2,999,039 9/1961 Lula 75-124 X temperatures 1700" F. and 1600" R). The results in- 3,108,870 10/ 9 Brady 7S-124 dicate that the steels of my invention do not tend to soften 3,251,583 5/ 1966 fl n 75-128 substantially upon prolonged aging.

From the foregoing it will be seen that the steels of my invention can be heat-treated to high strength by quenching from an elevated temperature such as about 1500 DAVID L. RECK, Primary Examiner. P. WEINSTEIN, Assistant Examiner. 

